The present invention relates to manhole assemblies and more particularly to manholes that house apparatus for coupling to below ground liquid storage tanks, principally liquid fuel storage tanks.
Conventional below ground storage tank assemblies require various coupling apparatus and ecological equipment to be stored below ground. Manholes are provided to house their assemblies for secure but easy access by the operator. The present practice in this country is to provide a manhole assembly comprising two main apparatus, namely, an at-grade manhole for at-grade access and protection and a below-grade manhole located partially within, but below the at-grade manhole cover.
Basic at-grade manhole elements include a cast metal top ring, a continuous skirt having its upper end connected to the ring and defining a chamber within which various apparatus can be located, and a manhole cover releasably resting on or within the top ring. The ring and skirt are embedded within the concrete or other roadway material with the upper edge of the ring and cover edges substantially flush with the roadway surface. The more modem manholes and below grade assemblies for gasoline storage tanks include a below-grade, flexible bellows type or bucket type housing the bottom of which is secured to the upper end of the tank riser pipe. The below-grade housing performs a number of important functions, such as providing chamber protection for coupling access to the top of the riser pipe and drop tube, containing fuel spills to prevent them from seeping into the ground, and at least temporarily containing vapors for atmospheric protection. Conventional below-grade manholes include a hatch cover that releasably locks and seals to the housing rim that defines the access hatchway to the housing interior. The hatch cover, in its closed position, is located a predetermined distance, usually 2-6 inches below the roadway surface.
Various technical problems have arisen from use of bellows and bucket housings in below-grade manholes. For example, the backfill and pea gravel for recently installed below ground storage tanks tend to settle thus lowering the riser pipe which in turn tends to lower the bellows or bucket housing by as much as 3 to 5 inches. To compensate, the operator must install a riser pipe extender to the top of the riser pipe. This extender decreases the distance between the top of the extender and the bottom of the hatch cover. Further settlement requires additional extenders or longer extenders. Eventually the operator cannot close the hatch cover because of extender interference. Normally, the operator leaves the hatch cover slightly open and un-sealed to rain water, humidity, and vapor escape rather than uninstall and remove the extender after each tank fill.
Another problem arises from operators who, after delivering fuel product to the storage tank, close the hatch cover onto the hatch rim and step on the top of the hatch cover with their foot to quickly seal and latch the hatch without bending down for hand closure. Placing an operator""s weight on to the hatch cover top depresses or collapses the bellows housing or lowers the bucket housing rim several inches. It is known that a certain percentage of bellows do not fully recover to their original height, thus causing the hatch rim to locate below design tolerance and interfere with the nozzle elbow of the tanker fill pipe and/or the closing of the hatch cover as described above.
A manhole assembly embodiment according to the principles of the present invention avoids the aforementioned problems and provides other benefits compared to the prior art by fixing the maximum vertical distance between the upper end portions of the below-grade manhole and the at-grade manhole. By fixing this distance, vertical movement of the riser pipe and/or drop tube shall not effect the closure of the hatch cover regardless of the need for drop tube extenders. Also, stepping forces on top of the hatch cover shall not cause any lowering of the top of the below-grade manhole upper end relative the at-grade manhole upper end.
In one exemplary embodiment, the hatchway rim of the below-grade housing is secured to the ring of the at-grade manhole. Another aspect of an embodiment includes making the distance between the ring and rim selectively adjustable. In one example, the cast iron rim defines at least three equally spaced bolt holes that vertically align when the at-grade manhole and below-grade manhole are properly aligned. Bolts and lock nuts can be used to adjust and securely fasten the at-grade manhole ring and below-grade manhole rim at a precise vertical spacing regardless of tank settling or downward forces applied to the top of the hatch cover. Since the top of riser pipe is threaded, an extender can be added to the top of the riser pipe to off-set any riser pipe or tank settling. Although, in the present embodiments, the riser pipe descends, the hatchway rim of the below-grade manhole does not, thus assuring reliable hatch cover closing.
In the event vertical adjustment of the below-grade hatchway rim is not desired, the invention contemplates welding or otherwise securing the rim to the at-grade manhole ring or skirt or welding ribs or stringers between the at-grade manhole ring or skirt and rim.